Great link, and a more specific answer than the one on IMDb. Thanks!
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Simon WhitakerDec 23 '12 at 11:11

1

I was in the military, and I never heard "zero dark thirty" used to refer specifically to 0030. It's slang for any obscenely early time in the morning.
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Nick StrehlkeDec 31 '12 at 6:51

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Just to clarify: I do think that this is the correct answer, in the sense that it directly relays what Bigelow meant with the title. However, I still believe that she herself got mixed up about the meaning of the term "zero dark thirty." By the way, here is another source—an unofficial dictionary for Marines written by a former Marine; 0 Dark 30 is the first entry. He even indicates in a note that the term was used (with the meaning that I'm familiar with) as early as the 1960s, when he served.
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Nick StrehlkeJan 2 '13 at 3:52

In military (US) slang that period [between midnight and dawn] is referred to as
"oh-dark hundred" or sometimes "zero-dark
hundred". On the 24-hour clock the hours before
10 am start with a 0; so 1:00 am is 0100 and said
as oh-one-hundred and so forth. Thus oh-dark
hundred is anytime after midnight while it is still dark:

"The woke us up at oh-dark-hundred and ran us
thru the obstacle course." Meaning they woke us
up in the wee hours of the morning before
daylight.

This answer is spot on. By the way, as I mentioned in a comment to another answer, the pronunciation "oh dark thirty" probably stems from service branch. In the Marine Corps, there was a lot of emphasis on pronouncing 0500 as "zero five hundred" instead of "oh five hundred," etc., and, as a result, we said "zero dark thirty" instead of "oh dark thirty."
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Nick StrehlkeDec 31 '12 at 6:57

Being in the Military I have rarely heard "Zero Dark Thirty" it is almost always pronounced as "Oh Dark Thirty," "Oh Dark Hundred" or mainly "Oh Dark Stupid". It refers to simply the 0 before the time in 24 hour time. Example:
0100, 0230, 0450 would all be pronounced in the above fashion and NOT by "One Dark Hundred" or "Two Dark Thirty". It is simply military slang or humour regarding getting woken up in the dark hours of the morning.

@Oliver_C No, the "zero" or the "oh" is more likely service branch-related. In the Marine Corps, we said "zero"; in the Army, they seem to say "oh."
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Nick StrehlkeDec 31 '12 at 6:50

I've been in both mech and light infantry (11B and 11H) as well as a 13D and never did we use that term (CONSUS or downrange). We NEVER used "oh" in place of "zero" as in "zero-one-thirty hrs" for 0130 hrs. When you conduct any sort of briefing or when issuing an OPORDER or a WARNO, never have I used the term nor seen or heard the term used. When you have to be somewhere as per issued orders at a certain time and place, you are told where and specifically when in EXACT terms, grid and SP times. If we were up at an ungodly hour in the morning we normally refer to it as "f***ing early in the mor
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user3787Jan 4 '13 at 10:45